Demand for faster welding speed and less spatter, so as to increase productivity, have been increasing in the welding industry. Faster welding speed increases production quantity per time, and thus welding productivity increases. Less spattering reduces a post-treatment process of removing spatter attached to a base material, and thus welding productivity also increases.
FIG. 5 shows waveforms of wire feed speed Wf, welding voltage Vw, and welding current Aw in conventional arc welding. First, conventional arc welding is described with reference to FIG. 5. In a known welding process, the wire feed speed is changed cyclically to forcibly cause a short circuit, and arc is re-generated by forcibly opening the short circuit (for example, refer to Patent Literature 1). In this prior art, the short circuit is opened without depending on electromagnetic pinch force of welding current, and thus spatter can be reduced.
In FIG. 5, time T1 is one time point in an arc period while arc is generated between a wire end and a base material. Wire feed speed Wf accelerates toward maximum speed Wf2.
At time T2, the wire and the base material are short-circuited, and a short circuit period starts. Wire feed speed Wf is controlled cyclically, regardless of an arc condition, according to a predetermined command value. Accordingly, a short-circuit timing may be a time point other than when wire feed speed Wf is at maximum speed Wf2. The short-circuit timing is a time point when the wire feed speed is around the maximum speed Wf2. It may be a time point during forward acceleration or forward deceleration. The timing differs with every short circuit.
Time T3, at which the short circuit is opened and arc is regenerated, comes during backward feed. A short-circuit opening timing may be a time point other than when wire feed speed Wf is at minimum speed (Wf4). The short-circuit opening timing is a time point when the wire feed speed is around minimum speed Wf4. It may be a time point during backward deceleration or backward acceleration. The timing differs with every short-circuit opening. At any timing, however, the short circuit is opened during backward feed. Accordingly, the short circuit is forcibly opened without depending on electromagnetic pinch force of welding current, and thus spatter can be reduced.
Wire feed speed Wf includes one forward feed and one backward feed in its one cycle. In one cycle, one short circuit and one opening of short circuit take place. In response to this cyclic operation of wire feed speed Wf, welding involving arc phenomenon is controlled. A predetermined cycle of wire feed speed Wf is a short-circuit generating frequency or the number of short circuits per second. This stabilizes welding while reducing spatter.
With respect to a welding device in which the wire is controlled to feed forward and backward, there is disclosed a welding control method of controlling the wire feed speed in response to welding phenomenon. (For example, refer to Patent Literature 2.) The wire feed speed is accelerated during the arc period, and is then controlled at a predetermined constant speed. When a short circuit is detected, the wire feed speed is decelerated, and then the wire is drawn up at a predetermined constant speed different from the above constant speed to open the short circuit and regenerate arc. Welding takes place through repetition of these operations. Also in this method, the short circuit is opened during backward feed. Accordingly, the short circuit is forcibly opened without depending on the electromagnetic pinch force of welding current, and thus spatter can be reduced.
In the aforementioned conventional welding control method disclosed in Patent Literature 1, stable welding with less spatter is achievable if there is no disturbance such as change of distance between the tip and base material. However, for example, if the position of base material deviates and the distance from the tip becomes longer during welding, the distance between the tip and base material becomes suddenly longer at timing A shown in FIG. 5. When this extended distance becomes greater than a distance advanced in the forward feed period of the wire feed speed, a short circuit does not occur. Then, the process goes on to backward feed in this state, which means the state without short circuit continues. Accordingly, generation of short circuit is delayed until the next forward feed period (e.g., until time T4). During this period without generation of short circuit, a droplet is formed at the wire end, and this droplet grows. A large droplet is released from the wire end by the movement of wire due to the change of distance between the tip and base material. This may become spatter and may splatter out of a weld pool. Even if the droplet does not splatter outside, a large droplet extends the short circuit state. The short circuit may not be sufficiently opened in the next short circuit, and thus the droplet may adhere to the base material. As a result, the state of unstable arc has occurred.
As shown in FIG. 6, let's say a short circuit occurs at time T5, and distance between the tip and base material becomes suddenly shorter at timing B. If this shortened distance becomes greater than a length of wire drawn up at the wire feed speed in the backward feed period, the short circuit continues without being opened until the next backward feed period (e.g., until time T6). In this case, temperature of a welded portion decreases, and weld bead narrows and thins due to extended short circuit time. This may result in uneven bead width. In addition, welding may become not feasible due to deposited wire end and base material. Alternatively, if a high current of about 400 A to 500 A is continuously applied, a short-circuiting wire portion may splatter by generating a large amount of spatter by means of electromagnetic pinch force, and arc may be regenerated. In any case, spatter generation increases, and the bead width becomes uneven.
In the welding device, in which the wire is controlled to feed forward and backward, disclosed in Patent Literature 2, the cycle of forward feed and backward feed of the wire feed speed is controlled in response to the arc phenomenon in the conventional welding control method for controlling the wire feed speed in line with the welding phenomenon. Accordingly, if the short circuit time becomes longer, the backward feed becomes longer. If the arc time becomes longer, the forward feed becomes longer. Opposite states are also controlled in the same way. The average feed speed of wire feed speed, short circuit cycle, and the number of short circuits become unstable and change if the arc phenomenon changes. Welding results thus cannot be stabilized.
If there is almost no change in the distance between the tip and base material, there is no problem. However, external disturbance such as change of distance between the tip and base material typically due to deviation in placement of base material or variations in accuracy of components, such as pressed components, frequently occur at actual production sites. Accordingly, the average feed speed of wire feed speed and short circuit cycle greatly change and fluctuate, resulting in difficulty to stabilize welding results.    Patent Literature 1: Japanese Patent Unexamined Publication No. S62-6775    Patent Literature 2: Japanese Patent Examined Publication No. S48-11463